American Gauge Design has established some standard dimensions for dial indicators for many years, and these include minimum and maximum diameters for a generally cylindrical housing, the dimensions of a bushing supporting the movable spindle, and the dimensions of a back mounting plate relative to the spindle bushing and lifter rod. Dimensional standardization for these components of indicators allows the industry to use clamps, mounts, and holders that can fix different indicators in place for different measurements, and a big investment has already been made in such devices.
Prior art indicators ordinarily use a rack-and-pinion drive for a pointer movable around a dial as the spindle moves relative to the housing, and there are many inherent inaccuracies and problems relative to such an arrangement. Electro-optical measuring systems using optically ruled glass scales and detectable moire fringe patterns are also generally known, and these are substantially more accurate and reliable than rack-and-pinion dial indicators. The invention seeks to use the improved accuracy and reliability of electro-optical measuring systems in an instrument that meets American Gauge Design dimensions requirements so as to be compatible with existing clamps, mounts, and holders. This creates dimensional problems if the spindle is to travel a substantial distance, because the required components for an electro-optical measuring system cannot be readily fit within the established dimensions for present dial indicators and allow much room for spindle travel.
The invention solves this problem with a special arrangement of components for an electro-optical measuring system within the established dimensional requirements for indicators while still providing a spindle travle of at least about 1/2 inch. The invention also aims at economy and reliability in meeting all the needs for an accurate electro-optical measuring system fitted within the present dimensional limitations of an indicator.